Multiple quantum wells are semiconductor structures comprised of alternating thin layers of two different semiconductor materials and, in particular, of semiconductor materials having differing bandgaps. Typically, layer thicknesses are of the order of 100 Angstroms and a typical structure might comprise 100 such layers, resulting in a total thickness of about 1 micrometer. Multiple quantum well structures are typically produced using well known epitaxy techniques, such as molecular beam epitaty or metal-organic chemical vapor deposition sometimes known as organometallic vapor phase epitaxy. Multiple quantum well structures have been used successfully in many different optical devices, such as optical modulators.
The quantum confined Stark effect (QCSE) has given rise to several innovations in electro-optic modulators. Such modulators have many applications in communications and special purpose computer systems. The principals behind the QCSE have been more fully explained by D. A. B. Miller et al, in Physics Review, 1985, B32, p1043. Briefly though, the QCSE is a phenomenon which arises when an electric field is applied across the plane of heterostructure superlattices. In a quantum well at zero electric field, the electron and hole energy levels are defined by the well width, and the electrons and holes are strongly confined in the well layer. However, when an electric field is applied, the electrons and holes are moved apart and their energies are altered. This has the effect of shifting the absorption resonance to lower energy as well as modulating the strength of the absorption. This occurs because direct optical absorption of a photon above the band gap energy involves raising an electron from one of the valence bands and putting it in the conduction band, otherwise known as formation of an electron-hole pair. This shift in the absorption resonance, then, provides for the optical modulation of any radiation that is incident to the heterostructure.
A typical structure for an such an optical modulator, also known as a quantum confined Stark effect (QCSE) modulator, is a p-i-n diode with the multiple quantum well structure formed within the intrinsic layer of the diode, i.e., the "i" region. In operation, a light beam is either directed perpendicular to the multiple quantum well layers or in the plane of the layers in a waveguide configuration, while at the same time a reverse bias is applied to the diode. Modulation of the lightbeam is effected by varying the reverse bias. An example of such a device is found in U.S. Pat. No. 5,105,301, issued on Apr. 14, 1992 to Campi and entitled, "Coupled Quantum Well Electro-optical Modulator."
In lieu of an electrically controlled multiple quantum well optical modulator, optical control may be employed. One such device, known as the self electro-optic effect device (SEED), essentially combines a multiple quantum well modulator with a photo detector. The photo detector generates the necessary electric field to change the absorption band of the multiple quantum well structure.
Examples of both electrically controlled and optically controlled multiple quantum well devices can also be found in an article by D. A. B. Miller, "Optoelectronic applications of quantum wells," Optics & Photonics News, vol. 1, no. 2, page 7, February 1990; U.S. Pat. No. 4,546,244, issued in October, 1985 to Miller; and U.S. Pat. No. 4,904,859, issued on Feb. 27, 1990 to Goossen et al and entitled, Self Electro-optic Effect Device Employing Asymmetric Quantum Wells."